1. Field of the Invention
The present invention relates to a coil bobbin, and particularly to a coil bobbin which undergoes a varnish impregnation treatment together with a magnetic core.
2. Description of the Related Art
A conventional transformer or choke coil comprises a magnetic core using a ferrite core and a plastic bobbin having a magnet wire wound therearound. The magnetic core consists of two separate sections like EE type, or UU type, where the two separate core sections abut against each other to form a closed magnetic path. In the structure, a gap may grow at the abutting contact portion and acts as a critical factor determining magnetic characteristics of the core. The abutting contact condition may be secured by adhesively bonding, taping or by means of a metallic spring.
The transformer or choke coil may undergo a well-known varnish impregnation treatment together with its magnetic core depending on its application. The treatment is to improve insulation performance of its winding section and to enhance stabilities against electrical oscillation and mechanical oscillation. The treatment is usually carried out such that the whole body of the transformer or choke coil except its terminal pins is immersed into a dilute solution of polyester resin, and then, is dried and cured at about 130 degrees C.
The plastic coil bobbin having a magnet wire wound therearound is formed of either a thermoplastic resin or a thermosetting resin, which is to be selected according to its application. Since the moisture resistance of the coil bobbin after the varnish impregnation treatment becomes an issue, the bobbin is formed of, for example, a thermoplastic resin with a low water absorption rate (polybutylene terephthalate), which is disclosed in Japanese Patent Publication No. Hei 11-335533.
In the coil bobbin described above, since a predetermined clearance is provided between the inner wall of a core housing portion 1 and a portion 2 (middle bar of an E core section) of a magnetic core 2 inserted in the core housing portion 1, a gap 4 exists inevitably therebetween as shown in FIGS. 6A to 6C showing cross sectional views of a conventional coil bobbin. Consequently, during the varnish impregnation treatment, varnish is allowed to penetrate into the gap 4 as well as the winding section. The cross sectional shape defined by inner wall surfaces of the core housing portion 1 and the cross sectional shape of the portion 2 are similar to each other, usually rectangular, with a slight proportional difference in dimension, and the gap 4 may be generated in three manners as shown in FIGS. 6A to 6C. Specifically, FIG. 6A shows that the portion 2 is not in contact with any of four inner wall surfaces of the core housing portion 1, FIG. 6B shows that the portion 2 has its one side surface brought into contact with one inner wall surface 30, and FIG. 6C shows that the portion 2 has its two adjacent side surfaces brought into contact with two inner wall surfaces 30 and 31. When the varnish is cured, the portion 2 is tightly fixed to the inside wall surface 30 in the case shown by FIG. 6B, and to the inside wall surfaces 30 and 31 in the case shown by FIG. 6C. Consequently, as shown in FIG. 7A, in case of a magnetic core comprising two portions, like an EE type and a UU type, brought into contact with each other at their abutting surface, the two portions 20 and 21 are forced to be tightly fixed to the inside wall surface(s) at either or both of the above-described abutting surfaces 30 and 31.
While the varnish impregnation treatment improves insulation performance of the winding section and also stabilities against electrical oscillation and mechanical oscillation, it creates the following problem. Since the two core sections 20 and 21 are tightly fixed, with varnish 40, to the inner surface of the core housing portion 1 as described above, a dimensional change of the coil bobbin due to changes in the ambient temperature or humidity generates stress at the fixation area causing force to act on an abutting contact surface G between the two core sections 20 and 21 which undergo a smaller dimensional change than the coil bobbin, thereby generating a gap g therebetween as shown in FIG. 7B. The gap g causes a magnetic reluctance to increase, resulting in reduced inductance of the transformer or choke coil.
Therefore, the coil bobbin is preferably formed of a resin material which undergoes least possible dimensional change due to changes in the ambient temperature and humidity. Particularly, to cope with the change due to the ambient humidity, a thermoplastic resin with a low water absorption rate (polybutylene-terephthalate) is preferably used. However, the coil bobbin formed of the thermoplastic resin (polybutylene-terephthalate) is easily deformed or its terminal pins are easily bent due to heat applied when the coil is subjected to soldering work, which causes a quality problem. Further, the soldering work requires a special caution, thereby hindering the working efficiency.